Preparation of chromium plating bath

ABSTRACT

DEPOSITION OF BRIGHT, DECORATIVE CHROMIUM PLATE IS OBTAINED FROM A CHROMIUM PLATING MEDIUM PREPARED AS A HOMOGENEOUS LIQUID BLEND OF WATER WITH DIPOLAR ORGANIC COMPOUND. THE MEDIUM IS ALSO MADE FROM A COMPLEX, WATER-SOLUBLE TRIVALENT CHROMIC COMPOUND FOR PLATING THAT CONTAINS CARBOXYLIC ACID CONSTITUENTS AND HALOGEN CONSTITUENTS AND EXHIBITS READY WATER SOLUBILITY. THE MEDIUM THUS PROVIDED CAN YIELD CHROMIUM DEPOSITS EXHIBITING ENHANCED PLATING COVERAGE.

United States Patent Ofiice 3,796,642 Patented Dec. 19, 1972 3,706,642 PREPARATION OF CHROMIUTVT PLAT'ING BATH James R. Brannan, Mentor, Ohio, assignor to E. I. du Pont de Nernours and Company, Wilmington, Del. No Drawing. Filed Feb. 19, 1971, Ser. No. 117,135

Int. Cl. C2313 5/06 US. Cl. 204-51 12 Claims ABSTRACT OF THE DISCLOSURE Deposition of bright, decorative chromium plate is obtained from a chromium plating medium prepared as a homogeneous liquid blend of water with dipolar organic compound. The medium is also made from a complex, Water-soluble trivalent chromic compound for plating that contains carboxylic acid constituents and halogen constituents and exhibits ready water solubility. The medium thus provided can yield chromium deposits exhibiting enhanced plating coverage.

BACKGROUND OF THE INVENTION Decorative chromium plating from baths made up with chromium in the trivalent state, wherein the baths are a blend of water and dipolar organic compound, has been shown, for example, in British Pat. No. 1,144,913. Such a bath, typically using a blend of water with dimethyl formamide and containing chromic chloride hexahydrate, has shown commercial promise. However, in working such baths, the most desirable performance in coverage of the chromium deposits, particularly at the low current density area, is not always achieved.

SUMMARY OF THE INVENTION It has now been found that in the plating of decorative chromium plate from media which are blended from water with dipolar organic compounds, that the media can be made up to provide chromium plate that is desirably enhanced, particularly in the low current density region.

Broadly, the method of the present invention is directed to preparing an electrolytic plating medium, for the plating of bright chromium plate, which method comprises establishing a homogeneous liquid mixture of water and dipolar organic compound, the proportion by volume of water to dipolar organic compound being within the range of 3 :1 to 1:3, blending with such medium a complex, water-soluble trivalent chromic compound containing carboxylic acid constituents and halogen constituents selected from the group consisting of chlorine, fluorine, bromine, iodine and mixtures thereof, with the chromic compound supplying such medium with a molar concentration of chromium within the range from about 0.5 to 3, and adjusting the pH of the medium within a range of between about l-4, while maintaining the temperature of the medium not substantially in excess of about 50 C.

The invention is further directed to methods of preparing chromium plating baths for the plating of bright decorative chromium plate by first preparing a complex, water-soluble chromic compound.

DESCRIPTION OF PREFERRED EMBODIMENTS The liquid medium containing complex, water-soluble chromic compound, is prepared as a homogeneous blend of water and dipolar organic compound. Preferably, the dipolar organic compound is aprotic thus minimizing hydrogen evolution at the cathode, for example during electrolytic plating operation. Suitable dipolar organic liquids which can or have been used, and which readily form homogeneous liquid mixtures with water, as well as thereby preparing a liquid medium containing the chromic plating compound in solution, include dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, bis (2-methoxyethyl)ether, sulfolane, tetramethylurea, and their mixtures. The dipolar organic compound is blended with water to provide a volume ratio of water to dipolar organic compound of between about 3:1 to 1:3 and preferably, best plating etficiency, e.g., enhanced conductivity and augmented plating coverage, the dipolar organic compound and water are present, by volume, in a ratio between about 3:2 to 2:3.

The complex, water-soluble chromic compound for preparing the plating medium contains carboxylic acid constituents plus halogen constituents which can be chlorine fluorine, bromine, iodine, or mixtures thereof. However, in typical commercial plating operation, bromine and iodine are not often used, for economy and to avoid evolution of visible noxious fumes at the anode. Thus chlorine and fluorine are preferred.

Although it is not meant that the complex, watersoluble chromic compound can contain carboxylic acid constituents that are representative of only a special group of carboxylic acids, such acids which can or have been used for the chromic compounds are typically exemplified by monocarboxylic and dicarboxylic acids, which may also contain hydroxyl groups, e.g., one or two such groups. For plating efficiency and water-solubility, advantageously these acids are non-aromatic acids containing less than about 10 carbon atoms; representative acids contain less than about six carbon atoms and include glycolic acid, lactic acid, and their mixtures, with preferably, for enhanced plating performance plus economy, some carboxylic acid constituents being supplied by glycolic acid. A compound of any of these acids such as a salt or an ester thereof, which acts in any of the reactions, such as those discussed in more detail hereinbelow whereby the complex is formed, in the same manner as the free acid, may also be used.

The plating media where the complex is formed in situ can be prepared by any of several methods. One method is the straightforward combination of chromium metal with carboxylic acid plus halogen-providing compound, for example hydrochloric acid. When such combination includes particulate chromium metal to reduce reaction time, the reaction can be highly exothermic, and therefore caution needs be taken in carrying out same. Typically for enhanced reaction efiiciency, as the reaction proceeds and the evolved heat starts to diminish, external heating is applied; and, where the reaction proceeds in aqueous medium such external heating can involve refluxing of the reaction mixture to augment completion of the reaction.

The complex of this type may also be prepared from the carboxylic acid and halogen-providing compound in admixture with chromic acid, typically charged to the reaction medium as a solution of chromic acid in water. In aqueous reaction medium, the chromic acid can be supplied by any of the suitable substances for forming chromic acid in water, e.g., chromium trioxide. The reaction resulting from this method is also exothermic and caution in the use of such method is also thus advisable.

As a further aspect of this method, chromic acid, for example, may be first reacted with halogen-providing compound such as hydrochloric acid to prepare a preformed reaction medium. To this medium carboxylic acid is added, preferably with heating to hasten completion of the reaction for final complex formation. In this reaction fugitive liquid can form a portion of the reaction medium, with water that may be supplied from the halogenproviding substance, e.g., a 20 to 37 weight percent solution of HCl in water providing the balance of the liquid medium. For the fugitive liquid, a secondary alcohol such as isopropanol is preferred, as taught in US. 2,524,803, and preferably one of no more than four carbon atoms, as such alcohol does not coordinate with the chromium and can be readily removed from the product by processes familiar in such art, e.g., evaporation with accompanying crystallization followed by water washing. After removal of fugitive liquid, the plating medium is then established as a homogeneous blend by the admixing of water and/ or dipolar organic compound as needed.

The complexes may further be prepared by reaction of chromic halide, with such halide corresponding to the halide that is to be present in the complex. When chromic halide is reacted with the carboxylic acid, this reaction further involves the addition of strong base, e.g., an alkali metal hydroxide yielding hydroxyl ions in the aqueous reaction medium. For example, CrF -9H O may be used in this method and will readily yield a chromium/carboxylic acid/fluoride complex involving exothermic reaction conditions.

After preparation of the complex, the plating medium thus or thereafter established, will thereby virtually always contain a molar ratio of total chromium atoms to total carboxylate constituent within the range of 1:07 to l 3.0, and further contain a molar ratio of total chromium atoms to all of the halogen atoms within the range of 110.1 to 1:3.5. Especially preferred ratios, based upon desirable plating performance and economy can depend upon the acid and also upon the halogen constituent. Thus for example, for a chromic carboxylate prepared with glycolic acid, and wherein such complex further contains chloride as the major amount, to all, of the halogen, the molar ratio of chromium atoms to halogen is preferably within the range of about 120.4 to 1:1. However when the halogen in such a complex is preponderantly to all fluoride, the molar ratio of chromium atoms to halogen atoms is preferably within the range of about 1:2.6 to 1:32. Further, the ratio of chromium to halogen can depend upon the method of complex preparation. For example in the above mentioned preparation method employing the CrF -9H O, the resulting complex will contain a molar ratio of chromium to moles of fluorine of 1:3. From this it can be appreciated that complex formation relying completely upon a chromic halide or a basic chromic halide, e.g., Cr(OH)Cl will always yield a complex wherein the molar ratio of moles of chromium to moles of halogen will be within the broad range of 1:07 to 1:3.

The complex is furnished to the plating medium in an amount to provide from about 25 to about 150 grams of chromium per liter, that is, the molar concentration of chromium in the plating medium is within the range from about 0.5 to about 3.0. The more highly concentrated baths having augmented viscosity are not well suited for deposition of chromium onto a substrate immersed therein. Thus such baths having a molar concentration of chromium above about 1.5 are used for spot plating, for example, brush plating. From the foregoing methods of preparation of the complex it can be appreciated that some unreacted, carboxylic-acid-supplying material, typically the free acid or an ester thereof, may be present with the chromic plating compound. It is permissible that such excess acid be present in the electrolytic plating medium and thereby form, together such other substances as may be present in their liquid state, a very minor amount of the total liquid of the liquid plating medium.

The plating media may also contain a salt of a strong acid preferably, for economy, an alkali metal salt; these salts enhance the conductivity achieved in the electroplating operation. Most preferably, for economy, the cation of the salt is sodium, potassium or their mixtures, and the strong acid anions should be halide anions, from an acid having a dissociation constant of at least K=10" for example chloride. The plating medium usually contains between about 30-150 grams per liter of such salts, basis liters of the plating medium. Such medium can also contain boric acid, or an equivalent to boric acid in aqueous solution, such as borax, boron oxide, or sodium oxyfluoborate. Such compounds may operate in the plating medium to augment the rate of deposition of the chromium and are typically used in amounts between about 10-70 grams per liter of the medium.

The pH of the plating medium should be maintained within the range from about 1 to 4 and preferably between about 2-3 to, for example, retard hydrogen evolution at the cathode during plating operation. The adjustment of pH can be typically readily achieved by the use of hydrochloric acid or an alkali metal hydroxide or carbonate, e.g., sodium hydroxide. The temperature of the medium can affect the useful plating range, such range being greater at lower temperature. Thus although plating may be carried out at a temperature as elevated as 50 C., a temperature below about 30 C. is almost always used.

During plating, the object to be plated is made the cathode, for example immersed in the plating medium, or the cathode in a spot plating system using a portable plating device supplying the electrolyte and a positive source of electrical current, e.g., brush plating operation, or the plating medium is contained in the brush and an inert anode is used such as a carbon, graphite, platinum or platinized titanium anode. Exemplary cathode substrates for receiving the plate include metals such as steel, brass, copper, copper alloys including bronze, zinc die castings and nickel. Additionally such plating can be performed on plastic surfaces which are activated or prepared for an electroplating operation.

When the deposition of the chromium plate is carried out in a bath, it may be carried out in any vessel useful for chromium electroplating such as tanks lined with corrosion resistant material including glass, ceramic material, polyvinyl chloride and the like. Also, electrodeposition with such plating baths may be performed by any conventional plating technique including rotating receptacle coating apparatus immersed in a plating bath. In selection of particular bath apparatus it is preferred that the anode immersed in the bath be made of graphite, for extended plating operation in a bath containing dimethyl formamide. For such baths, chlorine may form at the anode and build up in the bath causing early curtailment of continued plating operation. Preferably this is overcome by immersing the anode in an anolyte, and for this a diaphragm compartment cell is used in the plating operation. Such an anolyte can be an aqueous solution of typically an alkali metal acetate, e.g., sodium acetate, and this solution is separated from the plating solution or catholyte by the diaphragm. In the anolyte, for example one containing sodium acetate, the sodium acetate will be present in an amount of about 75 grams per liter.

The following example shows a way in which the invention has been practiced but should not be construed as limiting the invention. Plating tests in the example are conducted in a standard Hull cell. The standard Hull cell is a trapezoidal box of non-conductive material at the opposite ends of which are positioned anode and cathode plates, as has been more particularly described in US. Pat. No. 2,149,344. For the standard Hull cell, it is possible to easily determine the effective plating range of a plating composition under varying conditions. The current density at any point on a cathode is determined according to the formula A=C(27.7-48 log L) wherein A is the current density in amps per square foot (ASF) at the selected point, C is the total current in amps applied to the cell, and L is the distance in inches of the selected point from the high current density end of the plate.

EXAMPLE 1 Into a container there is placed 0.8 mole of chromium metal, 1.8 mole of glycolic acid of 70% strength, that is, 70% of glycolic acid and a balance of water, and 0.6

mole of 37.3% strength hydrochloric acid which is 37.3% by weight HCl in water. The container is covered and good ventilation is provided. After the ingredients are placed together in the container, dissolution of the chromium starts slowly but gradually increases thus supplying heat to the reaction. As the reaction continues the temperature of the reaction medium reaches about 70 C. without external heating and the chromium metal can be seen by visual inspection to be substantially dissolved. As the temperature starts to subside from about 70 (3., external heating is applied and the temperature of the reaction medium is permitted to reach 88 C. until all the chromium is dissolved. Total reaction time, i.e., to complete chromium metal dissolution, is about 4 hours. Thereupon the solution is heated at reflux, reaching a temperature of 107 C., for about 2 hours, and is thereafter permitted to cool.

The resulting complex, having a molar ratio of chromium to glycolic acid of 112.25 and of chromium t chloride of 1:0.75 is evaporated to form an ostensibly hydrated solid. Portions of this solid are dissolved in various blends of organic liquid and water, as shown in the table below, to prepare plating solutions for testing.

The particular Hull cell used has a capacity of 267 milliliters and uses graphite anodes, with the cathode for each test being a 3 75 by 2%" brass panel, each panel being nickel coated prior to use in the cell. As shown in the table below, each test is carried out using one ampere current for a 3-minute cycle and at a voltage as shown on the table below.

All panels plated Well at the high current density level, about 100 ASF for a panel plated at a current of one amp, so that the deviation in the plating range percentage on the panels was determined from observation of the plating deposit at the low current density level. Plating range as a percentage is that portion across the length of the panel on which a deposit is obtained, measured from the high current density area, divided by the total panel length and then multiplied by one hundred.

In the table below, abbreviations have been used to represent organic liquids employed with S representing sulfolane, TMU for tetramethylurea, DMF for dimethyl formamide, and DMA for dimethyl acetamide. For each experiment, the temperature of the plating solution and the pH, where such has been measured, are given in the table below along with the concentration of boric acid employed for each test. Where pH adjustment is necessary before plating, concentrated aqueous solutions of sodium hydroxide have been used.

results disclosed in such patent for such system. In addition to the substances listed above, this comparative bath contains one mole of sodium chloride and 0.6 mole of ammonium chloride. Such comparative bath can be expected to provide a plating deposit down to about 12 ASP. Compared to this, the plating baths prepared by the present invention can yield chromium deposits down to about 7 ASP under the plating conditions of this example.

What is claimed is:

1. The method of preparing an electrolytic plating medium for the plating of bright chromium plate which method comprises:

(1) establishing a homogeneous liquid medium of water and dipolar aprotic organic compound having a volume ratio of water to dipolar aprotic organic compound within the range of 3:1 to 1:3;

(2) blending with said medium a complex, watersoluble chromic compound containing halogen constituents selected from the group consisting of chloride, fluoride, mixtures thereof and mixtures with other halide, said chromic compound also containing carboxylic acid constituents supplying glycolic acid with said compound supplying said medium with a molar concentration of chromium within the range from about 0.5 to about 3 and said chromic compound having a molar ratio of chromium atoms to carboxylic acid constituents within the range of 1:07 to 1:3 and a molar ratio of chromium atoms to halogen atoms within a range of 110.1 to 1:3.5;

(3) adjusting the pH of said medium within a range of between about 1-4; while (4) maintaining the temperature of said medium not substantially in excess of about 50 C.

2. The method of claim 1 wherein said medium is furnished with between about 10-70 grams per liter of substance selected from the group consisting of boric acid, a substance supplying boric acid equivalent in aqueous solution, and mixtures thereof.

3. The method of claim 1 wherein said medium is furnished with between about -150 grams per liter of a salt of a strong acid having a dissociation constant of at least K=10" 4. The method of preparing an electrolytic plating medium for the plating of bright chromium plate, which method comprises:

(1) establishing a liquid, aqueous reaction medium containing chromium metal, carboxylic acid supplying glycolic acid constituents, and halogen-providing TABLE Plating range,

Organic Vol. H3303, percent liquid percent moles/l. pH F Amps Volts on panel 00 0.5 N.A. s0 1 23 70 40 0.5 3.0 so 1 11 e4 l Chromic chloride hexahydrate.

NOTE.N.A.=N0t available.

As can be seen from the above results, and focusing on the right hand column, plating from baths made in accompound supplying halogen constituents selected cordance with the present inventlon, and containing the from the group consisting of chloride, fluoride, mixchronuum glycolate complex, can ofler platmg coverage tures thereof, and mixtures with other halide, and

over a more greatly extended range, from a variety of organic liquids.

The comparative system has been prepared in accordance with the precepts of British Pat. No. 1,144,913 and in the above table yields a result consistent with the with suflicient of said carboxylic acid to provide said medium with a molar ratio of total chromium atoms to total acido groups within the range of 1:0.7 to 1:3, and with suflicient halogen-providing compound to supply said reaction medium with a molar ratio of total chromium atoms to total halogen atoms within the range of 1:0.1 to 1:35;

(2) permitting reaction of the substituents in said medium thereby preparing a resulting aqueous medium containing complex, water-soluble trivalent chromic compound containing carboxylic acid constituents and halogen constituents;

(3) blending dipolar aprotic organic compound with said resulting aqueous medium to prepare a homogeneous liquid medium having a volume ratio of water to dipolar aprotic organic compound within the range of 3:1 to 1:3, while adjusting the concentration of said complex in said liquid medium to supply not substantially above about 150 grams of chromium per liter of the medium;

(4) adjusting the pH of said medium within a range of between about 1.8 and about 4.9; and

(5) maintaining the temperature of said medium below about 50 C.

5. The process of claim 4 wherein said halogenproviding compound is selected from the group consisting of hydrogen fluoride, sodium fluosilicate, fluorine gas, hydrogen chloride, chlorine gas, and mixtures thereof.

6. The process of claim 4 wherein water supplies all of the liquid of said reaction medium, such medium is heated to boiling during reaction, and said chromium metal is in pulverulent form.

7. The process of preparing an electrolytic plating medium for the plating of bright chromium plate, which process comprises:

(1) bringing together in a liquid reaction medium, chromic acid, carboxylic acid supplying glycolic acid constituents, and halogen-providing compound supplying halogen constituents selected from the group consisting of chloride, fluoride, mixtures thereof, and mixtures with other halides, with suflicient of said carboxylic acid to provide said medium with a molar ratio of total chromium atoms to total acido groups within the range of 1:0.7 to 1:3, and with halogen-providing compound sufficient to supply said medium with a molar ratio of total chromium atoms to total halogen atoms within the range of 1:0.1 to 113.5;

(2) permitting reaction of the substituents in said medium, thereby preparing therein a complex, Watersoluble trivalent chromic compound containing carboxylic acid constituents and halogen constituents;

(3) removing fugitive liquid, if any, from said liquid reaction medium, thereby retaining a liquid medium consisting essentially of water, or water and dipolar aprotic organic compound;

(4) establishing said liquid medium as a homogeneous, blended liquid medium having a volume ratio of water to dipolar aprotic organic compound within the range of 3:1 to 1:3, while adjusting the concentration of said complex in said blended medium to supply not substantially above about 150 grams of chromium per liter of the medium;

(5) adjusting the pH of said medium within a range of between about 1.8 and about 4.9; and

(6) maintaining the temperature of said medium below about 50 C.

8. The process of claim 7 further characterized by a liquid reaction medium containing fugitive liquid that is volatilized from said medium after preparation of said complex.

9. The process of claim 7 wherein said carboxylic acid contains less than about 10 carbon atoms and is a nonaromatic acid selected from the group consisting of dicarboxylic acids, monocarboxylic acids, monocarboxylic and dicarboxylic acids containing at least one hydroxyl group, and mixtures thereof.

10. The process of claim 7 wherein said dipolar organic compound is an aprotic compound selected from the group consisting of dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, bis(2 methoxyethyl)ether, sulfolane, tetramethylurea, and mixtures thereof.

11. The process of preparing an aqueous electrolytic plating bath for the plating of bright chromium plate, which process comprises:

(1) establishing a liquid, aqueous reaction medium comprising chromium-providing material selected from the group consisting of chromic halide, basic chromic halide, and mixtures thereof, said halide constituents being selected from the group consisting of chloride, fluoride, mixtures thereof, and mixtures with other halide, said chromium providing material being further in mixture with carboxylic acid supplying glycolic acid constituents in amount sufficient to provide said medium with a molar ratio of total chromium atoms to total acido groups within the range of 1:0.7 to 1:3;

(2) permitting reaction of the substituents in said medium thereby preparing therein a complex, watersoluble trivalent chromic compound containing carboxylic acid constituents and halogen constituents;

(3) blending dipolar aprotic organic compound with said resulting aqueous medium to prepare a homogeneous liquid medium having a volume ratio of water to dipolar aprotic organic compound within the range of 3:1 to 1:3, while adjusting the concentration of said complex in said liquid medium to supply not substantially above about grams of chromium per liter of the medium;

(4) adjusting the pH of said medium within a range of between about 1.8 and 4.9; and

(5) maintaining the temperature of said medium below about 50 C.

12. The process of claim 11 wherein said reaction medium additionally contains a strong base yielding hydroxyl ions in said aqueous reaction medium when said chromic halide is present.

References Cited UNITED STATES PATENTS 1,922,853 8/ 1933 Kissel 204-51 2,517,441 8/1950 Raab 204-51 3,006,823 10/ 1961 Deyrup 20451 3,021,267 2/ 1962 Berzins 2045 1 FOREIGN PATENTS 697,225 9/1953 Great Britain 204-51 1,144,913 3/ 1969 Great Britain 20451 136,147 6/ 1960 Russia.

F. C. EDMUNDSON, Primary Examiner 

